This invention relates to a hydraulic tilt and trim mechanism for a marine propulsion unit and more particularly, to an improved hydraulic tilt and trim arrangement for such unit.
It is a well known practice in marine propulsion units, whether they are outboard motors or the outboard drive portion of an inboard-outboard drive, to mount the propulsion unit so that it can be adjusted for tilt and trim. These units normally incorporate a hydraulic motor that affects at least the trim and, at times, the tilt operation of the outboard motor. For convenience, the employment of hydraulic arrangements for moving the outboard drive both through the trim adjustment and for tilting the outboard drive up out of the water has been proposed. Normally, a reversible electric motor drives a reversible fluid pump that selectively pressurizes or depressurizes the hydraulic motor for effecting these movements.
The fluid pump sends pressurized fluid into various ports of a closed cylinder with a piston slidably mounted therein. The piston is sealed to the cylinder bore with a conventional seal, such as an O-ring, to prevent flow from between the separate chambers created by the piston within the cylinder. By pressurizing the chamber on one side of the piston and depressuring the other side the piston will move within the cylinder. Attached to the piston is a piston arm which is connected to the propulsion unit. The other end of the cylinder is attached to the transom of a watercraft. Thus, by pressurizing the chambers the operator of the boat changes the position of the piston thereby changing the relative angle of the propulsion unit to the watercraft.
The pressures in the cylinder chambers vary greatly depending on whether the propulsion unit is operating in the trim range or in the tilt range. In the tilt up range, usually associated with tilting the propulsion unit out of the water, the pump generates a relatively low pressure in the chambers because the only load on the cylinder is the weight of the propulsion unit. During the trim down mode the weight of the engine applies the requisite force to tilt down the engine.
Conversely, to trim up the motor the pump must generate far greater pressure because of the load placed on the unit by the propulsion unit. The increase in load is caused by the force created by the propeller of the propulsion unit moving the watercraft through the water. The relatively greater force created by the propulsion unit is directly transmitted into larger forces on the tilt and trim mechanism resulting in higher pressures required in the internal cylinders to trim up the motor in a running condition. In the trim down mode, the pump is not required to generate as much pressure as the driving force of the propulsion unit creates the trim down force.
Because of the need to relieve the pressure in the cylinder from the high pressure required in the trim up mode to the low pressure required in the tilt up mode a pressure relief device is needed. One suggested proposal for reducing the pressure in the tilt and trim cylinder during the transition between trim and tilt mode is shown in FIG. 9. This diagram shows a conventional hydraulic circuit and the piston design of a conventionally designed tilt and trim mechanism. The cylinder 19 contains two pistons: an upper piston 46 and a lower piston 45. The upper piston 46 is connected to the piston arm 23. The lower piston 45 abuts the upper piston 46. The cylinder has three openings for fluid to flow enter the internal cylinder chambers which are labeled as follows: upper cylinder chamber port 49, lower cylinder chamber port 48 and relief port 52.
In the trim mode, the lower edge of the lower piston is beneath the relief port 52. Thus, fluid can only enter or exit the lower cylinder chamber through port 48 in the trim mode. As stated above, during the trim up mode there is a relatively large pressure in cylinder chambers. When the lower edge of the lower piston rises above relief port 52 the system is in the tilt mode and the pressure required to trim the propulsion unit is no longer needed. Thus, the pressure is relieved by the fluid exiting the lower cylinder through the relief port 52.
To prevent fluid from seeping around the pistons from one chamber to another the piston are fitted with seals. The seals are typically O-rings made of a rubber compound. The O-rings provide adequate sealing as long as they are free from cuts or nicks. If they are damaged the fluid can pass around the piston preventing the tilt and trim mechanism from functioning properly.
One problem with the piston design, as shown in FIG. 9, is that the O-ring on the lower cylinder passes the relief port in the normal course of operation. Because the cylinder is typically made of metal and the relief port is typically a hole bored through the side wall of the cylinder the inner surface of the lower cylinder chamber in the area of the relief port may be rough. This rough inner surface may damage the O-ring seal of the piston resulting in durability problems.
It is therefore a principal object of this invention to provide an improved hydraulic trim control for a marine propulsion unit. It is a further objective of this invention to create a hydraulic trim and tilt control for a marine propulsion unit wherein the system is durable.